In a known manner a spiral, having turns of uniform section and pitch, via a particular conformation of the inside curve and the outside curve in the plane of the spiral or most often in different planes, enables one to obtain a concentric development of the spiral and a movement of the centre of the spiral weight and a variation in the spiral's inertia during development minimising working disruptions as a function of the amplitude and positions of the spiral with respect to the gravity vector. In addition to the fact that making such a spiral requires great skill, the space required in height constitutes a certain drawback for its use in wristwatches that have to have, for evident aesthetical reasons, the smallest possible thickness.
For this reason, use of a flat spiral is preferred, such as that shown in FIG. 1. Such a spiral is manufactured in a known manner by winding from a wire or metal band of constant section over its entire length, and has a constant pitch at rest between the turns. As can be seen in FIG. 1, the inside curve is fixed, for example by laser welding, onto a collet 20, driven onto the arbour 9 of a balance 8.
With respect to this state of the art, as regards the pitch between the turns, CH Patent No. 465 537, filed in 1966, should be mentioned, wherein there is disclosed a method for manufacturing spirals of any configuration, particularly with a variable pitch, from a metal strip or wire of constant section, wound in the groove of a die, then annealed and hardened. To the knowledge of the Applicant, no products of this type have been put on the market, which leads one to assume that the manufacturing method was not, technically or economically, satisfactory.
As regards the variation in thickness of a wound metal strip, GB Patent No. 1020 456 can be mentioned, which discloses the manufacture of a mainspring by buttwelding of strips having sections that increase from the centre to the periphery. Such a spring is designed, with equal space requirement, to increase the power reserve, but it is clear that by applying this manufacturing method to a spiral, the presence of welds would prevent a concentric development and would not allow reproducible isochronism to be obtained from one spiral to another.
This same principle had, moreover, already been proposed in U.S. Pat. No. 209,642 dating 1878, for improving the isochronism of a spiral made with an inside turn of smaller section. As will be seen in the detailed description, experiments contradict this assertion.